Controlled engine exhaust gas recirculation is a known technique for reducing oxides of nitrogen in products of combustion that are exhausted from an internal combustion engine to atmosphere. A typical EGR system comprises an EGR valve that is controlled in accordance with engine operating conditions to regulate the amount of engine exhaust gas that is recirculated to the fuel-air flow entering the engine for combustion so as to limit the combustion temperature and hence reduce the formation of oxides of nitrogen.
Because they are typically engine-mounted, EGR valves are subject to harsh operating environments that include wide temperature extremes and vibrations. Tailpipe emission requirements impose stringent demands on the control of such valves. An electric actuator, such as a solenoid that includes a sensor for signaling position feedback to indicate the extent to which the valve is open, can provide the necessary degree of control when properly controlled by the engine control system. An EGR valve that is operated by an electric actuator is often referred to as an EEGR valve.
Individual parts of an EGR valve must not only be strong, tightly toleranced, thermally insensitive, and essentially immune to combustion products present in engine exhaust gases, but they must also be cost-effective.
A cost-effective construction for the bearing in an EEGR valve comprises fabricating the bearing using powdered metal technology. A powdered metal bearing fabricated from stainless steel powder is well suited for use in the harsh environment of hot engine exhaust gases.
Increasingly stringent demands on valve performance, life, and reliability have led to the discovery that even small amounts of exhaust gas leakage through an EEGR valve bearing can be detrimental to valve performance over time.